A small amount of the cytochrome oxidase present in E. coli membranes can saturate the succinate dehydrogenase present to reconstitute succinoxidase activity. The succinate dehydrogenase, complexed with cytochrome b1 bands at an isopycnic density of 1.16 and a major component of cytochrome oxidase, has an isopycnic density of about 1.20. When these are recombined to make succinoxidase a new isopycnic species at p equals 1.18 is formed. The reconstituted succinoxidase is membranous and contains DCCD-sensitive ATPase but little or no outer membrane.